Katri Aarnio

Food and feeding habits of juvenile flounder Platichthys flesus (L.), abd turbot Scophthalmus maximus L. in the åland archipelago, northern Baltic Sea

Abstract The food choice of juvenile flounder ( Platichthys flesus ) and turbot ( Scophthalmus maximus ) was studied in the northern Baltic Sea during the years 1988, 1989, 1994 and 1995. The diet included organisms from 30 species/taxa in flounder (n = 306) and 10 species/taxa in turbot (n = 41). Flounder ⩽ 45 mm mainly consumed meiofauna (dominating taxon: Harpacticoida, Copepoda) and larger fish (46–101 mm) consumed macrofauna (dominating taxa: Oligochaeta, Amphipoda and Chironomidae). In terms of biomass, macrofauna dominated for all sizes of flounders, and meiofauna was important only for the smallest fish. A strong seasonal variation could be detected in the diet. In spring, macrofauna dominated for all size classes of fish (only fish > 30 mm were caught in spring), while in summer and autumn meiofauna dominated the diets for fish ⩽ 45 mm in size. Juvenile turbot (22–88 mm) consumed macrofauna and small fish. Turbot ⩽ 30 mm consumed mainly amphipods, while > 30 mm turbot consumed mysid shrimps, amphipods and fish. The ontogenetic shift from meio- to macrofauna-sized prey in flounders occurs at a larger fish size in the northern Baltic Sea than reported in other areas, possibly depending on the increased relative importance of meiofauna in the northern Baltic. The seasonal variation in the diet could be due to seasonally changing abundances in the zoobenthos, or for the small fish (1-group, spring), to switching from meio- to macrofauna in order to optimize their energy gain. The 0-group flounders consumed meiofauna for a long period, possibly due to a learning-process or simply due to easy availability of meiofauna. Turbot has a much larger mouth gap than flounders, thus allowing them to consume macrofauna from the beginning of their benthic life.

Some ecological properties in relation to eutrophication in the Baltic Sea

The current published information of the influence of eutrophication on the Baltic Sea is reviewed and summarized. Harmful effects at different levels of the ecosystem are identified, and the spatial and temporal variability of these properties characterized. The Baltic Marine Environment Bibliography was searched on the web, and some 1170 references with ‘eutrophication’ as a keyword were extracted and analyzed. The most studied regions were the Gulf of Finland (including the Archipelago Sea), Kattegat and the Bothnian Sea. The search was further divided into several parameters (transparency, oxygen/hypoxia, nutrients, primary production/ chlorophyll a, algal mats, macroalgae, zoobenthos and fish) related to eutrophication. In most regions, chlorophyll, zoobenthos and fish were the most commonly studied biological and ecological parameters. The linking of eutrophication, ecology and a potential decision-support system is discussed, and related to similar attempts elsewhere.

Seasonal variation in abundance and diet of the sand goby Pomatoschistus minutus (Pallas) in a northern Baltic archipelago

Abstract In shallow, soft-bottom benthic communities meiofauna is as important as macrofauna, with regard to both biomass and production. Shallow bottoms are also important as feeding habitats for the sand goby Pomatoschistus minutus and meiofauna accounts for a considerable part of their diet. Population dynamics, diet and potential predation effect on the infauna by P minutus were studied in a shallow (1—1.5 m) sandy area in the northwestern part of Aland, northern Baltic Sea (May-October, 1989).The predation efficiency of the fish was studied in aquaria with Ostracoda, Corophium volutator; Neomysis integer and Macoma balthica as prey. The abundance of P. minutus resembled that of other areas in the Baltic and neighbouring seas. The lowest density was found in October (0.5 ind/m2), while the highest abundance (11.2 ind/m2) was recorded in September. The diet of P. minutus varied seasonally and with increasing fish size. Small individuals ( 40 mm) a...

Predation by juvenile Platichthys flesus (L.) on shelled prey species in a bare sand and a drift algae habitat

Due to increasing eutrophication of the coastal Baltic waters, drifting algae are a common phenomenon. Drifting algal mats accumulate on shallow sandy bottoms in late summer and autumn, and affect the ambient fauna. Juvenile flounder, Platichthys flesus, utilize these habitats during their first few years. They feed on benthic meio- and macrofauna; part of their diet consists of shelled species, such as Ostracods, and juvenile Hydrobia spp. and Macoma balthica. Earlier studies have shown that up to 75% of ostracods and 92% of hydrobiids survive the gut passage of juvenile flounder, while all M. balthica are digested by the fish. We conducted laboratory experiments to study how the shelled prey responded to a drift algal mat, and the predation efficiency of juvenile P. flesus on these prey species on bare sand and with drifting algae (50% coverage). Hydrobia spp. utilized the drift algae as a habitat and, after 1 h, 50% had moved into the algae; ostracods and M. balthica were more stationary and, after 96 h, only 23 and 12%, respectively, were found in the algae. For the predation efficiency of P. flesus, a two-way ANOVA with habitat (algae, bare sand) and predation (fish, no fish) as factors revealed that both algae and predation affected negatively the survival of all three prey species. The algae, thus, affected the predation efficiency of juvenile P. flesus and the consumption of prey was much reduced in the algal treatments compared to the bare sand. This was due probably to increased habitat complexity and the ability of prey, especially hydrobiids, to use the algal mat as a refuge. Altered habitat structure due to drift algae, together with the resultant changes in habitat (refuge) value for different prey species, may profoundly change the structure of benthic communities.

Experimental evidence of predation by juvenile flounder, Platichthys flesus, on a shallow water meiobenthic community

In the northern Baltic Sea juvenile flounder, Platichthys flesus, occur in high abundances on shallow sandy bottoms in late summer and autumn. They feed mainly on meiofauna and the ontogenetic switch to macrofaunal sized food occurs at a larger size here than in other areas, exemplifying the high relative importance of meiofauna. Consequently, juvenile P. flesus in the Baltic feed for a longer period on meiofauna, and could thus be expected to have a stronger predation effect on the meiofaunal assemblages. In this study the predation effects of juvenile P. flesus on meiofaunal abundances and community structure were studied using microcosms that were sampled repeatedly over a 3-week period. Significant differences between treatment and control were found for the total number of taxa, for abundances of harpacticoids, copepod nauplii and ostracods. The nematode community was not affected, but one genus, Axonolaimus, was negatively affected by predation. The predation affected meiofaunal community structure as the major taxon diversity was significantly reduced. The results suggest that the meiofauna on shallow sandy bottoms may be structured by juvenile P. flesus, and the combined predation pressure of juvenile flounder and other epibenthic predators in the area might be considerable. Microcosms are effective in testing natural predation, especially on meiofaunal communities, but field experiments should be conducted to account for the physical characteristics of the area studied.

Nestedness of trophic links and biological traits in a marine food web

To understand the consequences of changes in diversity we need to consider the functional characteristics (traits) of species, as well as the trophic setting the taxa are part of. These two approaches have rarely been conducted in an integrated manner, although we know that trophic structure is an important driver of community functioning, and that biological traits, in particular body size, in turn determine which species interact. In this study, we assessed how structural food-web attributes (nestedness, generality, vulnerability) relate to multiple biological traits of interacting taxa. We found that the inherent complexity of a shallow subtidal trophic network of benthic macroinvertebrates and fish in the northern Baltic Sea contained identifiable and specific patterns: the feeding interactions were highly nested, both in terms of prey taxonomy and biological traits, suggesting trophic redundancy rather than trophic complementarity. Both trait diversity and trait redundancy of interacting species incr...

Mesograzer identity, not host algae, determines consumer stable isotope ratios

ABSTRACT Species-specific foraging habits or feeding preferences influence the overall trophic functioning in consumer assemblages. We set out to assess the in situ trophic diversity in a mesograzer assemblage (isopods, amphipods and gastropods) associated with marine littoral macroalgae. More specifically, we set out to establish whether stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) are dependent on primary consumer identity or expressed in relation to the identity of the algal host, i.e. the habitat of the consumer. Consumer locations in bivariate isotope space revealed significant differences among grazer taxa, but no effect of the algal host. This suggests that grazer-specific foraging is more important in driving the qualitative range of feeding in the consumer assemblage than host diversity per se within a macrohabitat, such as a bay or a lagoon. The stable isotope ratios of the mesoherbivores were in line with expectations based on the known feeding ecology of the grazers. However, the trophic diversity suggested by stable isotope analysis implies that even an established concept such as the mesograzer guild may encompass simplifications in terms of functional group membership.

The food web positioning and trophic niche of the non-indigenous round goby: a comparison between two Baltic Sea populations

The food web positioning of the non-native round goby (Neogobius melanostomus Pallas, 1814) was studied in a new invasive population in Mariehamn, Åland Islands (northern Baltic Sea). The trophic position and isotopic niche space was compared to other benthic-feeding fish species in the same habitat. The trophic position (TP) was estimated based on stable isotope analysis of carbon (13C:12C) and nitrogen (15N:14N) ratios and compared to that of an established invasive population in Hel, Gulf of Gdansk (southern Baltic Sea). Ontogenetic changes in isotope signatures were evaluated with a regression analysis and compared between the two populations. The round goby positioned as a second-order consumer on the third trophic level among other benthic-feeding fish. It showed a similar trophic position and significant isotopic niche overlap with large perch. The trophic position of round gobies in Mariehamn is significantly higher than in Hel, likely due to different prey items. Furthermore, the ontogenetic patterns differ between the two invasive populations, also suggesting differing resource and habitat availability and levels of intraspecific competition between areas.